The present invention relates to an optical system for optical pickup, an optical pickup apparatus, a coupling optical system, a coupling optical system lens and a recording/reproduction apparatus, and in particular, to an optical system for optical pickup, an optical pickup apparatus, a coupling optical system, a coupling optical system lens and a recording/reproduction apparatus which are used suitably for a high density optical disk apparatus employing a light source wherein monochromaticity is poor, or a wavelength fluctuates quickly.
In recent years, there has been developed, as an optical information recording medium, a DVD (storage capacity: 4.7 GB) which is similar in terms of size to a CD (storage capacity: 640 MB) and is enhanced in terms of recording density, and it has spread rapidly. For reproduction of the DVD, there is used a laser beam emitted from a light source wherein a wavelength is 635 nm-660 nm. A divergent light flux emitted from the laser light source is collimated to a collimated light flux by a collimator lens, and then, enters an objective lens whose numerical aperture (NA) on the part of DVD is 0.6 to be converged on an image recording surface through a transparent base board of DVD.
Recently, optical information recording media similar to CD and DVD having storage capacity of 10-30 GB are developed actively, by the use of an objective lens with further higher NA and a light source with further shorter wavelength. As a short wavelength light source which is considered to be hopeful, there is a GaN blue semiconductor laser having an oscillation wavelength of about 400 nm.
However, the oscillation wavelength of the GaN blue semiconductor laser is temperature-dependent in the same way as in other semiconductor lasers, and in addition, fluctuation of the wavelength is caused by the mode hop or by laser output, and high frequency superimposing is necessary for reducing laser noise, resulting in poor monochromaticity of the oscillation wavelength, which are specific characters. It is therefore predicted that correction of longitudinal chromatic aberration (longitudinal spherical aberration) is necessary in the light-converging optical system of high density optical disk employing the GaN blue semiconductor laser.
In Session WD 26 of ISOM/ODS"" 99 (Joint International Symposium on Optical Memory and Optical Data Storage 1999) Postdeadline Poster Papers, there was shown an example of experiments wherein, when a high density optical disk with storage capacity of 17 GB was reproduced by an ordinary objective lens whose numerical aperture on the part of a disk is 0.6, multi-mode oscillation is caused by high frequency superimposing to make the spectral band width to be 0.7 nm (FWHM), and jitters were worsened, compared with an occasion where no high frequency superimposing was carried out, and there was proposed an objective lens of a junction doublet type wherein longitudinal chromatic aberration is improved, and the surface on the part of a light source and the surface on the part of a disk are aspherical, and the junction surface is spherical.
As an objective lens of a junction doublet type for an optical disk wherein chromatic aberration is corrected, those described in Japanese TOKKAISHO Nos. 61-3110 and 62-286009 are known. Further, Japanese TOKKAIHEI No. 9-311271 discloses an objective lens wherein both surfaces are aspherical and represent a phase type diffraction element, and a numerical aperture on the part of an optical information recording medium is 0.7 or more.
There are further proposed an optical disk device employing an objective lens of a two-element structure whose numerical aperture on the part of an optical information recording medium is 0.85, and a high density disk. For example, in Session WC 1 (on page 50 of the preliminary report) of ISOM/ODS"" 99, there was announced an optical disk device having storage capacity of 9.2 GB wherein a red semiconductor laser with wavelength of 65.0 nm was used. Further, in Session WD 1 (on page 28 of the preliminary report 2) of ISOM/ODS"" 99, it was announced that the storage capacity of 20 GB was attained by using a GaN semiconductor laser with wavelength of 400 nm. With regard to these semiconductor lasers, there was no announcement about an influence of wavelength fluctuation.
In Japanese TOKKAIHEI No. 11-174318, there is described an objective lens of a two-element structure wherein the numerical aperture on the part of an optical information recording medium is 0.85 and a hologram is provided on the optical surface for correcting chromatic aberration. As stated above, it is possible to correct chromatic aberration by introducing a diffraction element in the objective lens optical system, but, manufacturing cost of a diffraction lens made of glass is very high in general.
On the other hand, as an objective lens made of plastic wherein chromatic aberration is corrected by a diffraction element, those wherein the numerical aperture on the part of an optical information recording medium is about 0.55 are put to practical use. However, with regard to an objective lens made of plastic, its spherical aberration fluctuates when a refractive index of the objective lens is varied by temperature change. Therefore, the objective lens made of plastic is not suited for a converging optical system of a high density optical disk wherein a short wavelength is required for a light source and a high NA is required for the objective lens. Though spherical aberration fluctuation caused by temperature change can be lightened by the two-element structure even in the case of an objective lens made of plastic, change in an outside diameter caused by temperature change is great because the coefficient of expansion of plastic material is greater than that of glass, and thereby, the lens tends to be distorted by temperature change to low temperature or to high temperature after being incorporated in the lens frame, which is a problem. Therefore, when the spherical aberration fluctuation caused by temperature change also is considered, it is preferable that at least one of two lenses is made to be a glass lens.
Being affected by heat generation from an actuator for driving an objective lens, temperature distribution is caused on the objective lens, and this causes distribution of refractive index when the objective lens is a plastic lens, thus, wavefront aberration of a light flux that has passed through the objective lens is worsened to cause problems easily. Accordingly, even in the case of an objective lens of a two-element structure, it is preferable that both of the two elements are represented by a glass lens when temperature characteristics are taken in consideration. However, the glass lens has a problem of manufacturing cost as stated above, and it is actually difficult to collect chromatic aberration by only an objective lens for high density disk.
As represented by an objective lens of a junction doublet type for optical disk, a lens wherein a lens made of low dispersion material having positive refracting power and a lens wherein a lens made of high dispersion material having negative refracting power are combined without using a diffraction element, is not suited for an objective lens for optical disk which is definitely required to be light in weight. The reason for the foregoing is that the refracting power of each lens is required to be great to attain high NA of the lens because dispersion of a material is limited, thereby, weight of the lens itself tends to be increased.
Japanese TOKKAISHO No. 62-269922 discloses a method to correct longitudinal spherical aberration of the total light-converging optical system for recording/reproducing an optical information recording medium, by correcting chromatic aberration of an objective lens insufficiently and by correcting chromatic aberration of a collimator lens excessively.
In view of the circumstances mentioned above, an object of the invention is to provide an optical pickup optical system which can be manufactured at low cost and can be corrected in terms of longitudinal spherical aberration by a relatively simple structure even when using a light source with poor monochromaticity or a light source wherein a wavelength varies quickly, in high density optical disk device, for example, an optical pickup device equipped with the aforesaid optical system for optical pickup and a recording/reproducing apparatus equipped with the optical pickup device.
Further object of the invention is to provide a coupling optical system which can correct longitudinal spherical aberration when it is used, for the purposes of correction of spherical aberration, correction of sine conditions, making to be small, making to be thin, making to be light and making to be low cost, together with an objective lens wherein longitudinal spherical aberration remains, because of a large numerical aperture on the part of an optical information recording medium, and to provide a lens for the coupling optical system. Another object of the invention is to provide a coupling lens for an optical pickup device and an optical pickup device which are capable of recording/reproducing information properly, even when a laser light source unit such as one to house two lasers, for example, in one package is used.
First, there will be considered the reasons why standards for longitudinal spherical aberration are strict in high density disk, and why longitudinal and remaining chromatic aberration in the vicinity of wavelength of a light source in an objective lens optical system used for a light-converging optical,system for recording/reproducing an existing optical information recording medium, matters.
In the following explanation, let it be assumed that xcex represents a wavelength of a light source and NA represents a numerical aperture of an objective lens optical system on the part of an optical information recording medium. To make the explanation simple, let is be assumed that a light-converging optical system is composed of a light source, a collimator, an objective lens optical system and a transparent base board of an optical information recording medium, and the collimator is an ideal lens (all aberrations are corrected). Namely, even when the wavelength of the light source fluctuates, an incident light flux into the objective lens optical system is a collimated light flux that is in parallel with an optical axis.
Further, with regard to the objective lens optical system, its spherical aberration is corrected completely for standard wavelength xcex0, and root mean square value Wrms of wavefront aberration is 0 under the condition of focus. Let is be assumed that spherical aberration is not changed by an objective lens optical system for wavelength fluctuation xcex94xcex, but back focus fB is changed by xcex94fB. When the objective lens optical system is driven by fB in the direction of an optical axis for focusing, for back focus variation, Wrms is 0, but when no focusing is conducted, Wrms is expressed by following Expression (1).
Wrms=0.145xc2x7{(NA)2/xcex}/|xcex94fB|xe2x80x83xe2x80x83(1)
From Expression (1), when DVD (NA=0.6, xcex=650 nm) is compared with advanced optical disk having NA 0.85 and xcex=400 nm as an example, deterioration of wavefront aberration for the latter is 3.26 times that of the former even when xcex94fB is the same for both cases. Namely, if the tolerance of the wavefront aberration is the same, the tolerance of |xcex94fB| is made to be as small as ⅓.26, which makes it necessary to lessen the remaining longitudinal spherical aberration.
In the case of a single thin lens, when assuming that a focal length is represented by f and a refractive index is represented by n(xcex) as a function of wavelength xcex, the following expression (2) holds.
df/dxcex=[xe2x88x92f/{n(xcex)xe2x88x921}]xc2x7dn/dxcexxe2x80x83xe2x80x83(2)
When n(xcex+xcex11) and n(xcexxe2x88x92xcex12) are introduced to replace a differential with a finite difference, the expression (2) can be rewritten as the following expression (3). In this case, xcex11 and xcex12 are assumed to represent an amount of deviation from the standard wavelength.
xcex94f=[xe2x88x92f/{n(xcex)xe2x88x921}]xc2x7{n(xcex+xcex11)xe2x88x92n(xcexxe2x88x92xcex12)}=f/xcexd(xcex,xcex11,xcex12)xe2x80x83xe2x80x83(3)
In the expression above, however, the following expression holds;
xcexd(xcex,xcex11,xcex12)={n(xcex)xe2x88x921}/{n(xcexxe2x88x92xcex12)xe2x88x92n(xcex+xcex11)}
wherein, xcexd(xcex,xcex11,xcex12) represents dispersion under the condition of wavelength xcex.
In the case of a photographic lens that deals with natural light, there are used xcex=xcexD, xcexxe2x88x92xcex12=xcexF and xcex+xcex11=xcexC (xcexD, xcexF and xcexC are refractive indexes for spectral lines respectively of d line (589.6 nm), f line (486.1 nm) and c line (656.3 nm)). In the light-converging optical system for recording/reproducing an optical information recording medium, fluctuation in wavelength of a light source and longitudinal spherical aberration in a range of the fluctuation are important. In the present specification, therefore, it is assumed that xcex11=xcex1=10 nm and a unit of xcex is represented by nm, and xcexd(xcex) is defined again to be like the following expression (4).
xcexd(xcex)={n(xcex)xe2x88x921}/{n(xcexxe2x88x9210)xe2x88x92n(xcex+10)}xe2x80x83xe2x80x83(4)
Because of fB=f in the thin lens, the expression (3) turns into the following expression;
xcex94fB=f/xcexd(xcex)xe2x80x83xe2x80x83(5)
wherein xcex94fB shows a rough value of a change in back focus corresponding to a wavelength variation of 20 nm in the vicinity of the wavelength.
(Table 1) shows refractive indexes at 780 nmxc2x110 nm, 650 nmxc2x110 nm and 400 nmxc2x110 nm for main lens materials. The wavelength of 780 nm is one used for CD, 650 nm is one used for DVD and 400 nm is one that is expected to be used for an optical disk of advanced as mentioned earlier.
Table 2 shows results of calculations of xcexd(780), xcexd(650) and xcexd(400) conducted from Table 1 and based on the expression (4).
As is apparent from Table 2, the shorter the wavelength is, the smaller is xcexd(xcex), and from the expression (5), the shorter the wavelength is, the greater is xcex94fB. In this case, xcexd(400) is as small as xc2xc to ⅕ of xcexd(650) as shown in Table 3.
Accordingly, when an objective lens is represented by a refraction lens whose longitudinal spherical aberration is determined by xcexd(xcex) of a lens material, fluctuation of back focus caused by wavelength variation is too big to reduce the longitudinal spherical aberration sufficiently, in application of an advanced optical disk where a blue semiconductor laser having a wavelength of about 400 nm is used.
Incidentally, a diffraction element can control a light flux by means of diffraction, and thereby, has a lens effect which is the same as that of a refraction lens. The relationship between focal length f of the diffraction element and wavelength of a light source can be expressed by the following expression (6) when of represents a focal length in the case of designed wavelength xcex0.
f=(xcex0/x)ofxe2x80x83xe2x80x83(6)
When the expression (6) is changed in a form like a refraction lens, there holds the following expression which further turns into the expression that follows the following expression.
xe2x80x83df/dxcex=xe2x88x92(xcex0/xcex2)of=xe2x88x92f/xcexxe2x80x83xe2x80x83(7)
                                                                        Δ                ⁢                                  xe2x80x83                                ⁢                f                            =                                                -                                      (                                          f                      /                      λ                                        )                                                  ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                λ                                                                                        =                                                -                  f                                ⁢                                                      {                                                                  (                                                  λ                          +                          α1                                                )                                            -                                              (                                                  λ                          -                          α2                                                )                                                              }                                    /                  λ                                                                                                        =                              f                ⁢                                                      {                                                                  (                                                  λ                          -                          α2                                                )                                            -                                              (                                                  λ                          +                          α1                                                )                                                              }                                    /                  λ                                                                                        (        8        )            
When the following expression holds in the same way as in the refraction lens;
xcex94f=f/xcexd(xcex,xcex11,xcex12)xe2x80x83xe2x80x83(9)
the following expression holds.
xcexd(xcex,xcex11,xcex12))=xcex/{(xcexxe2x88x92xcex12)xe2x88x92(xcex+xcex11)}xe2x80x83xe2x80x83(10)
Now, it is assumed that xcex11=xcex12=10 nm and a unit of xcex is represented by nm, and xcexd(xcex) is defined again to be like the following expression (11).
xcexd(xcex)=/{(xcexxe2x88x9210)xe2x88x92(xcex+10)}xe2x80x83xe2x80x83(11)
Therefore, the results of calculation of xcexd(780), xcexd(650) and xcexd(400) concerning a diffraction element based on the expression (11) are as follows.
xcexd(780)=xe2x88x9239.0
xcexd(650)=xe2x88x9232.5
xcexd(400)=xe2x88x9220.0
It is possible to correct longitudinal spherical aberration, while controlling the refractive power of each element to be small by combining the diffraction element having negative xcexd(xcex) with a diffraction element having positive xcexd(xcex), because xcexd(xcex) of the diffraction element is negative as shown above,
However, when xcexd(400) is compared with xcexd(650) concerning a diffraction element, a change of the absolute value is small when it is compared with a refraction lens, although the absolute value itself becomes small. Therefore, in the blue semiconductor laser area, correction of chromatic aberration is considered to be difficult even when a diffraction element is combined with a refraction element.
Next, there will be examined a light-converging optical system composed of an objective lens optical system composed of an infinite conjugate type refraction single lens whose longitudinal spherical aberration is determined by xcexd(xcex) and of a collimator optical system composed of a hybrid element wherein a diffraction element is combined with a refraction element, or of a coupling optical system.
An infinite conjugate type objective lens is a lens that is corrected so that its spherical aberration through a transparent base board of an optical information recording medium for a collimated light flux that is in parallel with an optical axis may be mostly minimum. Namely, magnification m0 of the objective lens is zero. Therefore, the coupling optical system turns into a collimator optical system which changes a light flux emitted from a light source into a collimated light flux which is in parallel with an optical axis and is corrected in terms of spherical aberration.
Concerning the light-converging optical system like one stated above, when longitudinal spherical aberration of an objective lens is represented by xcex94fBO, and longitudinal spherical aberration of a collimator optical system is represented by xcex94fBC, longitudinal spherical aberration xcex94fBC of the total light-converging optical system is expressed by the following expression, when mT represents the magnification of the total light-converging optical system.
xcex94fBT=xcex94fBO+(mT)2xc2x7xcex94fBCxe2x80x83xe2x80x83(12)
In the occasion where longitudinal spherical aberration of the total light-converging optical system is corrected, xcex94fBT is zero. Therefore, in this case, the expression (12) is changed in terms of a form, and the following expression holds.
xcex94fBC=xe2x88x92(1/mT2)xc2x7xcex94fBOxe2x80x83xe2x80x83(13)
In this case, the objective lens and the collimator optical system are analyzed as a thin lens. Since the back focus is equal to the focal length in the thin lens, the following expressions hold.
xe2x80x83xcex94fBO=xcex94ofxe2x80x83xe2x80x83(14)
xcex94fBC=xcex94fCxe2x80x83xe2x80x83(15)
Therefore, the expression (13) can be substituted with the following expression.
xcex94fC=xe2x88x92(1/mT2)xc2x7xcex94ofxe2x80x83xe2x80x83(16)
Even for the collimator optical system, calculation will be advanced under the condition of approximation to a thin type wherein a diffraction element and a refraction element are arranged to be close to each other. When a focal length of the diffraction element is represented by fCR, a focal length of the refraction element is represented by fCD and a focal length of the total collimator optical system is represented by fC, the following expression holds.
1/fC=1/fCR+1/fCDxe2x80x83xe2x80x83(17)
When both sides of the above expression are differentiated, the following expression holds.
xe2x88x92xcex94fC/fC2=xe2x88x92xcex94fCR/fCR2xe2x88x92xcex94fCD/fCD2xe2x80x83xe2x80x83(18)
Therefore, when the expression (5) is considered, the following expression holds.
xcex94fC=(xcex94fCR/fCR2+xcex94fCD/fCD2)xc2x7fC2={1/(xcexdRxc2x7fCR)+1/(xcexdDxc2x7fCD)}xc2x7fC2xe2x80x83xe2x80x83(19)
In the expression above, xcexdR represents dispersion of a material of the refraction element, namely, it is xcexd(xcex), while, xcexdD represents dispersion of the diffraction element.
Therefore, the conditional expression under which the longitudinal spherical aberration should be corrected is as follows, from the expressions (16) and (19).
{1/(xcexdRxc2x7fCR)+1/(xcexdDxc2x7fCD)}xc2x7fC2=xe2x88x92(1/mT2)xc2x7xcex94fOxe2x80x83xe2x80x83(20)
In the aforesaid expression, fC and mT are based on specifications of the light-converging optical system. xcex94fO is a remaining longitudinal spherical aberration of the objective lens, and when the objective lens is a refraction single lens, xcex94fO is determined by a focal length of the refraction single lens and by dispersion xcexd(xcex) of a lens material. Further, xcexdD is a constant determined in accordance with a wavelength, and xcexdR is an amount that is determined if a material of the lens element constituting the collimator optical system is selected.
Therefore, for obtaining unknowns fCR and fCD, simultaneous equations need to be solved. Though a process of calculation is omitted here, fCR and fCD can be expressed as follows.
fCR=(xcexdRxe2x88x92xcexdD)xc2x7fC/[xcexdR{1+xcexdDxc2x7xcex94fO/(mT2xc2x7fC)}]xe2x80x83xe2x80x83(21)
xe2x80x83fCD=(xcexdDxe2x88x92xcexdR)xc2x7fC/[xcexdD{1+xcexdRxc2x7xcex94fO/(mT2xc2x7fC)}]xe2x80x83xe2x80x83(22)
Now, typical two examples will be examined by utilizing the expressions (21) and (22) stated above. First, there will be taken the example where longitudinal spherical aberration of the objective lens optical system is already corrected. Because of xcex94fO=0 at this time, the expressions (21) and (22) are as follows.
fCR={(xcexdRxe2x88x92xcexdD)/xcexdR}xc2x7fCxe2x80x83xe2x80x83(23)
fCD={(xcexdDxe2x88x92xcexdR)/xcexdD}xc2x7fCxe2x80x83xe2x80x83(24)
The expressions (23) and (24) respectively show fCR and fCD for an individual collimator optical system composed of a hybrid element wherein a diffraction element and a refraction element are combined, on which longitudinal spherical aberration is corrected.
Now, let power (refracting power) representing an inverse number of a focal length be introduced. When assuming that xcfx86C represents power of the total collimator optical system, xcfx86CR represents power of the refraction element constituting the collimator optical system and xcfx86CD represents power of the diffraction element constituting the collimator optical system, there hold the following expressions.
xcfx86C=1/fCxe2x80x83xe2x80x83(25)
xe2x80x83xcfx86CR=1/fCRxe2x80x83xe2x80x83(26)
xcfx86CD=1/fCDxe2x80x83xe2x80x83(27)
xcfx86C=xcfx86CR+xcfx86CDxe2x80x83xe2x80x83(28)
From the expressions (23), (25) and (26), the following expression holds;
xcfx86CR={xcexdR/(xcexdRxe2x88x92xcexdD)}xc2x7xcfx86Cxe2x80x83xe2x80x83(29)
and from the expressions (24), (25) and (27), the following expression holds.
xcfx86CD={xcexdD/(xcexdDxe2x88x92xcexdR)}xc2x7xcfx86Cxe2x80x83xe2x80x83(30)
Further, following expressions hold when the expressions (29) and (30) are deformed.
xcfx86CR/xcfx86C=xcexdR/(xcexdRxe2x88x92xcexdD)xe2x80x83xe2x80x83(31)
xcfx86CD/xcfx86C=xcexdD/(xcexdDxe2x88x92xcexdR)xe2x80x83xe2x80x83(32)
Further, when a wavelength of a light source is 400 nm, xcexdD=xe2x88x9220 is induced from the expression (11), and xcexdR=165 is induced from Table 2 when olefine resin is used as a lens material. Therefore, when these numerical values are substituted in the expressions (31) and (32), following expressions hold.
xcfx86CR/xcfx86C=0.892xe2x80x83xe2x80x83(33)
xcfx86CD/xcfx86C=0.108xe2x80x83xe2x80x83(34)
Next, there will be considered an occasion wherein remaining longitudinal spherical aberration for an objective lens xcex94fO is not zero. In the case of an objective lens which is a refraction single lens, when the objective lens is assumed to be a thin lens, the following expression holds in the same way as in the expression (3).
xcex94fOxcex1=fO/xcexdO(xcex)xe2x80x83xe2x80x83(35)
Further, in the case where longitudinal spherical aberration of an objective lens optical system representing a refraction single lens made of a material with dispersion xcexdO(xcex) is not corrected, when the objective lens is assumed to be a thin lens, following expression (36) holds.
fC=xe2x88x92fO/mTxe2x80x83xe2x80x83(36)
Therefore, the following expression holds.                                                                         Δ                ⁢                                  xe2x80x83                                ⁢                                  fO                  /                                      (                                                                  mT                        2                                            ·                      fC                                        )                                                              =                                                -                                      (                                          1                      /                      mT                                        )                                                  ·                                  (                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          fO                      /                      fO                                                        )                                                                                                        =                                                -                  1                                /                                  {                                      mT                    ·                                          vO                      ⁡                                              (                        λ                        )                                                                              }                                                                                        (        37        )            
When the expression (37) is substituted in the expressions (21) and (22), following expressions hold.
fCR=(xcexdRxe2x88x92xcexdD)xc2x7fC/[xcexdR{1xe2x88x92xcexdD/(mTxc2x7xcexdO(xcex))}]xe2x80x83xe2x80x83(38)
fCD=(xcexdDxe2x88x92xcexdR)xc2x7fC/[xcexdD{1xe2x88x92xcexdR/(mTxc2x7xcexdO(xcex))}]xe2x80x83xe2x80x83(39)
When calculation is conducted by the use of the expressions (25)-(27), for the expressions (38) and (39), following expressions hold.
xe2x80x83xcfx86CR/xcfx86C=xcexdRxc2x7{1xe2x88x92xcexdD/(mTxc2x7xcexdO(xcex))}/(xcexdRxe2x88x92xcexdD)xe2x80x83xe2x80x83(40)
xcfx86CD/xcfx86C=xcexdDxc2x7{1xe2x88x92xcexdR/(mTxc2x7xcexdO(xcex))}/(xcexdDxe2x88x92xcexdR)xe2x80x83xe2x80x83(41)
The expressions (40) and (41) show that xcfx86CR/xcfx86C and xcfx86CD/xcfx86C are determined not by a focal length of the objective lens optical system but by xcexdR, xcexdD, xcexdO(xcex) and mT, when correcting longitudinal spherical aberration for the total light-converging optical system combined with an objective lens optical system wherein a collimator optical system composed of a hybrid element wherein a diffraction element and a refraction element are combined.
For example, when a wavelength of a light source is made to be 400 nm in the same way, the expression xcexdD=xe2x88x9220 holds, and when olefine resin is used as a material of the collimator optical system, the expression of xcexdR=165 holds from Table 2. In this case, with regard to xcexdO(400)=254, namely, FCD1 representing a low refractive index and low dispersion material described in Table 2 and xcexdO(400)=101, namely, M-NbFD82 representing a low refractive index and low dispersion material described in Table 2, the relationship between mT and xcfx86CR/xcfx86C and relationship between mT and xcfx86CD/xcfx86C were calculated based on the expressions (40) and (41).
In the light-converging optical system for recording/reproducing an optical information recording medium, mT ranging from xe2x88x92xc2xd to xe2x88x92xe2x85x9 is usually used (for reproducing only, mT ranging from xe2x88x92⅕ to xe2x88x92xe2x85x9 is usually used, but for both reproducing and recording, mT ranging from xe2x88x92xc2xd to xe2x88x92⅕ is used because it is necessary to take in, efficiently as much as possible, a divergent light flux emitted from a light source) So, there were calculated xcfx86CR/xcfx86C and xcfx86CD/xcfx86C for the occasion where mT takes xe2x88x92xc2xd (xe2x88x920.500), xe2x88x92⅓ (xe2x88x920.333), xe2x88x92xc2xc (xe2x88x920.250), xe2x88x92⅕ (xe2x88x920.200), xe2x88x92⅙ (xe2x88x920.167), xe2x88x92{fraction (1/7)} (xe2x88x920.143), xe2x88x92xe2x85x9 (xe2x88x920.125) and xe2x88x92{fraction (1/20)} (xe2x88x920.050). Results of them are shown in Table 4 and Table 5.
As is apparent from Table 4 and Table 5, diffraction power becomes stronger as mT approaches 0. In the case of xcexdO=254, xcfx86CR/xcfx86C is negative when mT is xe2x88x92{fraction (1/20)}. On the other hand, in the case of xcexdO=101, xcfx86CR/xcfx86C falls below zero when mT exceeds xe2x88x92⅕, and an absolute value of xcfx86CR/xcfx86C becomes greater while taking the negative value as mT further approaches zero. Further, when mT is xe2x88x92{fraction (1/20)}, an absolute value of each of xcfx86CR/xcfx86C and xcfx86CD/xcfx86C exceeds 1, and even in this case, it is possible to manufacture a diffraction optical element used for a coupling optical system, because a numerical aperture of a collimator optical system is small, as will be stated later.
The relationship between numerical aperture NAC of the collimator optical system on the part of a light source and numerical aperture NAO of the objective lens optical system on the part of an optical information recording medium is expressed by the following expression (42).
xe2x80x83NAC=xe2x88x92mTxc2x7NAOxe2x80x83xe2x80x83(42)
When it is assumed that NACR represents a numerical aperture of a refraction lens and NACD represents a numerical aperture of a diffraction lens, the following expressions hold.
NACR=NACxc2x7xcfx86CR/xcfx86Cxe2x80x83xe2x80x83(43)
NACD=NACxc2x7xcfx86CD/xcfx86Cxe2x80x83xe2x80x83(44)
Values of NACR and NACD for NAO=0.85 were obtained based on the expressions (42) and (44) to be tabulated in Table 4 and Table 5.
Minimum pitch xcex9min of a diffraction pattern by a diffraction optical element can approximate as shown by the following expression (45), when a wavelength is xcex. Therefore, Table 4 and Table 5 show also the value of xcex/NACD.
xcex9min=xcex/NACDxe2x80x83xe2x80x83(45)
As is apparent from Table 4 and Table 5, NACR takes a value of 0.32 or less, NACD takes a value of 0.20 or less and xcex9min takes a value of 2 xcexcm or more, and these values are all manufacturable values. Since a light flux is not interfered actually even when an objective lens optical system is driven in the direction perpendicular to an optical axis of a collimator optical system for tracking, NAC, NACR and NACD are larger than the values shown in Table 4 and Table 5.
It is also possible to make a diffraction-refraction integrated optical element wherein one side or both sides of a refraction lens are made to be a diffraction optical element to be a coupling optical system, without constituting the coupling optical system with a refraction lens and a diffraction lens which are separate each other. As a diffraction-refraction integrated optical element, those having NA of about 0.55 wherein longitudinal spherical aberration of a light-converging optical system for recording/reproducing an optical information recording medium is corrected, are proposed as an objective lens. In the invention, for the reason that a numerical aperture of a refraction lens is small and curvature of the optical surface is not so large, and for the reason that a request for a coupling optical system to be thin and small for insurance of a working distance and light weight is not so strong unlike an objective lens, it is possible to manufacture a lens of a diffraction-refraction integrated optical element easily and at low cost as a part of the coupling optical system.
The object stated above can be attained by either one of the following Structures (1)-(20).
Structure (1): An optical pickup apparatus for recording and/or reproducing information on an optical information recording medium comprising a light source emitting a light flux, a light-converging optical system to converge a light flux emitted from the light source, having therein a coupling optical system which changes a divergent angle of a light flux emitted from the light source and an objective lens optical system which converges a light flux emerging from the coupling optical system on an information recording surface of the optical information recording medium, and an optical detector for detecting reflected light or transmitted light coming from the information recording surface of the optical information recording medium.
In the optical pickup apparatus stated above, at least one surface of the coupling optical system is provided with a diffraction surface by which the longitudinal spherical aberration on the occasion wherein light having a wavelength differing from that of light with a prescribed wavelength by a prescribed difference of wavelength is made to enter the objective lens optical system through the coupling optical system is made to be smaller, compared with the longitudinal spherical aberration on the occasion wherein light having a wavelength differing from that of light with a prescribed wavelength from that of the light having the prescribed wavelength by a prescribed difference of wavelength is made to enter the objective lens optical system through the coupling optical system.
Structure (2): The optical pickup apparatus according to Structure (1), wherein when light having a wavelength shorter than that of light having the prescribed wavelength is made to enter the objective lens optical system, a position of a focus is on the under side compared with a focus in the case of the light with the prescribed wavelength, and when light having a wavelength shorter than that of light having the prescribed wavelength is made to enter the coupling lens optical system, a position of a focus is on the over side compared with a focus in the case of the light with the prescribed wavelength.
Structure (3): The optical pickup apparatus according to Structure (1), wherein when light having a wavelength shorter than that of light having the prescribed wavelength is made to enter the objective lens optical system, a position of a focus is on the over side compared with a focus in the case of the light with the prescribed wavelength, and when light having a wavelength shorter than that of light having the prescribed wavelength is made to enter the coupling lens optical system, a position of a focus is on the under side compared with a focus in the case of the light with the prescribed wavelength.
Structure (4): The optical pickup apparatus according to Structure (1), wherein the objective lens optical system does not have a diffraction section on the optical surface.
Structure (5): The optical pickup apparatus according to Structure (1), wherein the objective lens optical system is composed of one two-sided aspherical surface lens.
Structure (6): The optical pickup apparatus according to Structure (1), wherein the objective lens optical system is composed of two lenses which have at least one aspherical surface refraction surface.
Structure (7): The optical pickup apparatus according to Structure (1), wherein a numerical aperture of the objective lens optical system on the part of an optical information recording medium is 0.58 or more.
Structure (8): The optical pickup apparatus according to Structure (7), wherein a numerical aperture of the objective lens optical system on the part of an optical information recording medium is 0.65 or more.
Structure (9): The optical pickup apparatus according to Structure (1), wherein a wavelength of the light source is 700 nm or less.
Structure (10): The optical pickup apparatus according to Structure (9), wherein a wavelength of the light source is 600 nm or less.
Structure (11): The optical pickup apparatus according to Structure (1), wherein the coupling optical system is a collimator optical system which converts a light flux emitted from the light source into a light flux which is in parallel substantially with an optical axis.
Structure (12): The optical pickup apparatus according to Structure (1), wherein the coupling optical system makes a divergent angle of a light flux emitted from the light source to be smaller.
Structure (13): The optical pickup apparatus according to Structure (1), wherein the coupling optical system converts a light flux emitted from the light source into a converged light.
Structure (14): The optical pickup apparatus according to Structure (1), wherein the diffraction surface has a diffraction pattern which is almost in a shape of concentric circles, and a minimum pitch xcex9min in the diffraction pattern satisfies the following conditional expression;
2xcex less than xcex9min less than 100xcex
wherein xcex represents a wavelength of a light flux emitted from the light source.
Structure (15): The optical pickup apparatus according to Structure (14), wherein a wavelength of the light source is not more than 450 nm, the diffraction surface has a diffraction pattern which is almost in a shape of concentric circles, and a minimum pitch xcex9min in the diffraction pattern satisfies the following conditional expression.
2xcex less than xcex9min less than 30xcex
Structure (16): The optical pickup apparatus according to Structure (1), wherein the diffraction surface has a diffraction pattern which is almost in a shape of concentric circles, and a minimum pitch xcex9min in the diffraction pattern satisfies the following conditional expression.
0.4 xcexcm less than xcex9min less than 13.5 xcexcm
Structure (17): The optical pickup apparatus according to Structure (1), wherein a focal length of the coupling optical system for light having a wavelength of 400 nm is in a range from 2 mm to 25 mm.
Structure (18): The optical pickup apparatus according to Structure (1), wherein the coupling optical system has a plastic lens.
Structure (19): A coupling lens used in an optical pickup apparatus comprising a diffraction surface, wherein when light having a wavelength that is shorter than that of light having a prescribed wavelength is made to enter the coupling lens, a position of a focus is on the over side compared with a focus in the case of the light having the prescribed wavelength.
Structure (20): An apparatus for recording and/or reproducing an optical information recording medium which records and/or reproduces information on an optical information recording medium comprising an optical pickup apparatus having a light source emitting a light flux, a light-converging optical system for converging a light flux emitted from the light source including a coupling optical system that changes a divergent angle of a light flux emitted from the light source and an objective lens optical system that converges a light flux emerging from the coupling optical system on the information recording surface of the optical information recording medium, and an optical detector for detecting reflected light or transmitted light coming from the information recording surface of the optical information recording medium, wherein at least one surface of the coupling optical system is provided with a diffraction surface by which the longitudinal spherical aberration on the occasion wherein light having a wavelength differing from that of light with a prescribed wavelength by a prescribed difference of wavelength is made to enter the objective lens optical system through the coupling optical system is made to be smaller, compared with the longitudinal spherical aberration on the occasion wherein light having a wavelength differing from that of light with the prescribed wavelength from that of the light having the prescribed wavelength by the prescribed difference of wavelength is made to enter the objective lens optical system through the coupling optical system.
Further, preferable Structures (21)-(48) are as follows.
Structure (21) of the invention is represented by an optical pickup optical system comprising a coupling optical system that changes. a divergent angle of an incident light and an objective lens optical system that converges a light flux coming from the coupling optical system, wherein at least one surface of the coupling optical system is provided with a diffraction surface by which the longitudinal spherical aberration on the occasion wherein light having a wavelength differing(from that of light with a prescribed wavelength)by a prescribed difference of wavelength is made to enter the coupling optical system through the objective lens optical system is made to be smaller, compared with the longitudinal spherical aberration on the occasion wherein light having a wavelength differing from that of light with a prescribed wavelength by a prescribed difference of wavelength is made to enter the objective lens optical system.
As stated above, in the optical pickup optical system of Structure (21), a diffraction surface is provided on at least one surface of the coupling optical system. It is therefore possible to correct longitudinal spherical aberration generated by the objective lens optical system by the coupling optical system, if there is made an arrangement that a sign of the longitudinal spherical aberration generated by the diffraction surface is opposite to that of the longitudinal spherical aberration generated by the refraction surface of the objective lens optical system, and an amount of the former longitudinal spherical aberration is (1/mT)2 times that of the latter longitudinal spherical aberration. Since the coupling optical system does not have various limitations about the objective lens optical system stated above, it is possible to manufacture the coupling optical system through injection molding, by using resin materials such as plastics. Therefore, it is possible to manufacture an optical pickup optical system easily and at low cost.
In short, the optical pickup optical system of Structure (21) makes it possible to correct longitudinal spherical aberration easily and at low cost, even in the case of using a light source having poor monochromaticity like GaN blue semiconductor laser with a wavelength of about 400 nm, or a light source whose wavelength fluctuates suddenly.
Structure (22) is represented by an optical pickup optical system wherein the objective lens optical system does not have a diffraction surface on its optical surface.
Structure (23) is represented by an optical pickup optical system wherein the objective lens optical system is composed of a two-sided aspherical lens.
Structure (24) is represented by an optical pickup optical system wherein the objective lens optical system is composed of two lenses in which at least one aspherical refraction surface is included.
Structure (25) is represented by an optical pickup optical system which can be applied to the optical pickup optical system described in either one of Structures (21)-(24).
Structure (26) is represented by an optical pickup apparatus comprising at least one light source, a coupling optical system that changes a divergent angle of a divergent light emitted from the light source, and an objective lens optical system that converges a light flux emerging from the coupling optical system on the information recording surface of an optical information recording medium, wherein at least one surface of the coupling optical system is provided with a diffraction surface by which the longitudinal spherical aberration on the occasion wherein light having a wavelength differing(from that of light with a prescribed wavelength)by a prescribed difference of wavelength is made to enter the coupling optical system through the objective lens optical system is made to be smaller, compared with the longitudinal spherical aberration on the occasion wherein light having a wavelength differing from that of light with a prescribed wavelength by a prescribed difference of wavelength is made to enter the objective lens optical system.
Structure (27) is represented by an optical pickup apparatus wherein the objective lens optical system does not have a diffraction surface on its optical surface.
Structure (28) is represented by an optical pickup apparatus wherein the objective lens optical system is composed of a two-sided aspherical lens.
Structure (29) is represented by an optical pickup apparatus wherein the objective lens optical system is composed of two lenses in which at least one aspherical refraction surface is included.
Structure (30) is represented by an optical pickup apparatus wherein a numerical aperture of the objective lens optical system on the part of an optical information recording medium is 0.58 or more.
Structure (31) is represented by an optical pickup apparatus wherein a numerical aperture of the objective lens optical system on the part of an optical information recording medium is 0.65 or more.
Structure (32) is represented by an optical pickup apparatus wherein a wavelength of the light source is 700 nm or less.
Structure (33) is represented by an optical pickup apparatus wherein a wavelength of the light source is 680 nm or less.
Structure (34) is represented by an optical pickup apparatus wherein the coupling optical system is a collimator optical system which converts an incident divergent light flux into a collimated light flux that is substantially in parallel with an optical axis.
Structure (35) is represented by an optical pickup apparatus wherein the coupling optical system converts an incident divergent light flux into a light flux having a smaller divergent angle.
Structure (36) is represented by an optical pickup apparatus wherein the coupling optical system converts an incident divergent light flux into a convergent light flux.
Structure (37) is represented by an optical pickup apparatus wherein the diffraction surface has a diffraction pattern which is almost in a shape of concentric circles and minimum pitch xcex9min in the diffraction pattern is a value satisfying 2xcex less than xcex9min less than 100xcex with a wavelength of the light source represented by xcex, in which xcex represents a wavelength in the light source.
Structure (38) is represented by an optical pickup apparatus wherein wavelength xcex in the light source is not more than 450 nm, the diffraction surface has a diffraction pattern which is almost in a shape of concentric circles and minimum pitch xcex9min in the diffraction pattern is a value satisfying 2xcex less than xcex9min less than 100xcex.
Structure (39) is represented by an optical pickup optical system which can be applied to the optical pickup apparatus described in either one of Structures (26)-(38).
Structure (40) is represented by a coupling optical system which can be applied to the optical pickup optical system described in Structure (39).
Structure (41) is represented by a coupling optical system lens wherein a diffraction surface having a diffraction pattern which is almost in a shape of concentric circles is provided on at least one side thereof, and minimum pitch xcex9min of the diffraction pattern is a value satisfying 2xcex less than xcex9min less than 30xcex for the wavelength xcex of 450 nm or less.
Structure (42) is represented by a coupling optical system lens wherein a diffraction surface having a diffraction pattern which is almost in a shape of concentric circles is provided on at least one side thereof, and minimum pitch xcex9min of the diffraction pattern is a value satisfying 0.4 xcexcm less than xcex9min less than 13.5 xcexcm.
Structure (43) is represented by a coupling optical system lens wherein a wavelength which makes diffraction efficiency to be the maximum is not more than 450 nm.
Structure (44) is represented by a coupling optical system lens wherein focal length f is a value satisfying 2 mm less than f less than 25 mm when light having wavelength xcex of 400 nm is made to enter the coupling optical system lens.
Structure (45) is represented by a coupling optical system lens which is made of plastic.
Structure (46) is represented by a coupling optical system lens wherein a diffraction surface is formed through a photopolymerization method.
Structure (47) is represented by a lens having on at least one side thereof a diffraction surface having a diffraction pattern which is almost in a shape of concentric circles, wherein an absolute value of the longitudinal spherical aberration is greater and a sign thereof is opposite when compared with those of a refraction lens which has the same material, same axial thickness, same focal length and same back focus as those of the aforesaid lens and does not have a diffraction surface.
Structure (48) is represented by a coupling optical system wherein the lens described in Structure (47) is provided.
Structure (49) is represented by a recording/reproducing apparatus which is equipped with an optical pickup apparatus described in either one of Structures (26)-(39) and can record or reproduce at least one of a sound and an image.
Further, preferable Structures (50)-(83) are as follows.
Structure (50) is represented by an optical pickup apparatus for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the first light source and the second light source are unitized, and a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens.
Since a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, even when the first light source and the second light source are unitized, and positions of the two light sources are fixed to prevent the two light sources from moving separately in the direction that is in parallel with an optical axis of the light-converging optical system, it is possible to correct chromatic aberration with a diffraction effect of the diffraction pattern, and thereby to conduct excellent recording/reproducing of information. Incidentally, unitization means that the first light source and the second light source are housed and fixed in one package, for example, but without being limited to this, the unitization also include broadly condition where two light sources are fixed to be unable to correct aberration.
As a light source with the first wavelength and a light source with the second wavelength, there are considered various ones such as a blue laser light source and a red laser light source. Further, when there exist three light sources each having a different wavelength, two of them have only to be in the relationship stated above. As a coupling lens, there are also considered, in addition to a single lens, a two-element lens wherein an ordinary lens and a plate member on which a diffraction pattern which hardly has power, for example, are combined, a cemented lens, and a hybrid lens. As a material for the coupling lens, resin is preferable for forming a diffraction pattern, although glass is also considered. The light-converging optical system also includes one which is provided with not only a lens but also a mirror.
Structure (51) is represented by an optical pickup apparatus for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis. Incidentally, the optical axis of the light-converging optical system in the present specifications means an axis passing through the center of the coupling lens, because when the light-converging optical system has therein a mirror, an angle of the optical axis varies in accordance a reflection angle of the mirror. The principal ray of light means a central ray of light of the light flux restricted by an aperture such as a diaphragm.
Since the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis, even when the light source on one side is arranged at the position where the light flux is not emitted in the direction that is in parallel with an optical axis of the light-converging optical system, the light flux can be emitted in the direction of the optical axis as if such light source is on the optical axis of the light-converging optical system, resulting in better recording/reproducing of information. Incidentally, in Structure (51), it is preferable that the diffraction surface is not made of a ring zone-shaped diffraction surface but a stepwise stripe type diffraction surface.
Structure (52) is represented by an optical pickup apparatus wherein a diffraction pattern is formed on at least one surface of the coupling lens, and therefore, a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system can be made to emerge in the direction that is almost in parallel with an optical axis.
Structure (53) is represented by an optical pickup apparatus for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the coupling lens has off-axis characteristics to correct off-axis coma in the light-converging optical system for the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source. Incidentally, coma is a standard deviation value of wavefront aberration.
Essentially, it is preferable to design so that off-axis-optical characteristics of the light-converging optical system are made to be close to the axial optical characteristics as far as possible, to be ready for occurrence of a shifted axis caused by assembling accuracy. However, when a light source is away from the optical axis, a principal ray of light of a light flux emerging from the coupling has an inclination from an optical axis, and coma on a certain level is caused by passage through an objective lens and a transparent base board. Therefore, if it is possible to give inverse coma so that the coma may be canceled by the coupling lens, it is possible to reduce coma of the total light-converging optical system effectively for the light flux incident obliquely from the optical axis of the light-converging optical system, and thereby, to conduct excellent recording/reproducing of information. Incidentally, in the Structure (53), it is preferable that coma is corrected at the diffraction surface.
Structure (54) is represented by an optical pickup apparatus which is preferable because its structure can be simplified and its cost may be lowered when the first light source and the second light source are unitized.
Structure (55) is represented by an optical pickup apparatus for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the first light source and the second light source are unitized, and a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, and the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis.
Since a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, even when the first light source and the second light source are unitized, and positions of the two light sources are fixed to prevent the two light sources from moving separately in the direction that is in parallel with an optical axis of the light-converging optical system, it is possible to correct chromatic aberration with a diffraction effect of the diffraction pattern, and thereby to conduct excellent recording/reproducing of information. Further, since the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis, even when the light source on one side is arranged at the position where the light flux is not emitted in the direction that is in parallel with an optical axis of the light-converging optical system, the light flux can be emitted in the direction of the optical axis as if such light source is on the optical axis of the light-converging optical system, resulting in better recording/reproducing of information.
Structure (56) is represented by an optical pickup apparatus wherein the coupling lens is a single lens.
Structure (57) is represented by an optical pickup apparatus wherein only one surface of the coupling lens is provided with a diffraction pattern, and a diffraction effect of the diffraction pattern makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system to emerge in the direction that is almost in parallel with the optical axis.
Structure (58) is represented by an optical pickup apparatus wherein a diffraction pattern is provided on each of both sides of the coupling lens, and an effect of diffraction of diffraction patterns on the both sides corrects chromatic aberration, and makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system to emerge in the direction that is almost in parallel with the optical axis.
Structure (59) is represented by an optical pickup apparatus wherein the coupling lens has diffraction patterns on its both sides, and an effect of diffraction of the diffraction pattern on one side corrects chromatic aberration, while, an effect of diffraction of the diffraction pattern on the other side makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system to emerge in the direction that is almost in parallel with the optical axis.
Structure (60) is represented by an optical pickup apparatus for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the first light source and the second light source are unitized, a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, and the coupling lens has off-axis characteristics to correct off-axis coma in the light-converging optical system for the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source.
Since a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, even when the first light source and the second light source are unitized, and positions of the two light sources are fixed to prevent the two light sources from moving separately in the direction that is in parallel with an optical axis of the light-converging optical system, it is possible to correct chromatic aberration with a diffraction effect of the diffraction pattern, and thereby to conduct excellent recording/reproducing of information. Further, since the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis, even when the light source on one side is arranged at the position where the light flux is not emitted in the direction that is in parallel with an optical axis of the light-converging optical system, the light flux can be emitted in the direction of the optical axis as if such light source is on the optical axis of the light-converging optical system, resulting in better recording/reproducing of information. Furthermore, coma on a certain level is caused by passage through an objective lens and a transparent base board on off-axis light which causes a certain image height. Therefore, if it is possible to give inverse coma so that the coma may be canceled by the coupling lens, it is possible to reduce coma of the total light-converging optical system effectively for the light flux incident obliquely from the optical axis of the light-converging optical system, and thereby, to conduct excellent recording/reproducing of information.
Structure (61) is represented by an optical pickup apparatus wherein the wavelength xcex1 and the wavelength xcex2 have the relationship of xcex1 less than xcex2, and the relationship of t1 less than t2 holds when a thickness of a transparent base board of the first optical information recording medium is represented by t1 and a thickness of a transparent base board of the second optical information recording medium is represented by t2, thus, recording/reproducing of information can be conducted on different recording media such as, for example, CD and DVD.
Structure (62) is represented by an optical pickup apparatus wherein a light flux emitted from the second light source enters obliquely for an optical axis of the light-converging optical system.
Structure (63) is represented by an optical pickup apparatus wherein the coupling lens is a collimator which converges a incident divergent light flux to be a collimated light flux which is almost in parallel with an optical axis.
Structure (64) is represented by an optical pickup apparatus coupling lens for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis.
Since the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light-source to emerge in the direction that is almost in parallel with the optical axis, even when the light source on one side is arranged at the position where the light flux is not emitted in the direction that is in parallel with an optical axis of the light-converging optical system, the light flux can be emitted in the direction of the optical axis as if such light source is on the optical axis of the light-converging optical system, resulting in better recording/reproducing of information.
Structure (65) is represented by an optical pickup apparatus coupling lens wherein a diffraction pattern is formed on at least one surface of the coupling lens, and thereby, an effect of diffraction can make a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system to emerge in the direction that is almost in parallel with the optical axis.
Structure (66) is represented by an optical pickup apparatus coupling lens for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the coupling lens has off-axis characteristics to correct off-axis coma in the light-converging optical system for the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source.
Essentially, it is preferable to design so that off-axis optical characteristics of the light-converging optical system are made to be close to the axial optical characteristics as far as possible, to be ready for occurrence of a shifted axis caused by assembling accuracy. However, when a light source is away from the optical axis, a principal ray of light of a light flux emerging from the coupling has an inclination from an optical axis, and coma on a certain level is caused by passage through an objective lens and a transparent base board. Therefore, if it is possible to give inverse coma so that the coma may be canceled by the coupling lens, it is possible to reduce coma of the total light-converging optical system effectively for the light flux incident obliquely from the optical axis of the light-converging optical system, and thereby, to conduct excellent recording/reproducing of information.
Structure (67) is represented by an optical pickup apparatus coupling lens which is preferable because its structure can be simplified and its cost may be lowered when the first light source and the second light source are unitized.
Structure (68) is represented by an optical pickup apparatus coupling lens for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the first light source and the second light source are unitized, and a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, and the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis.
Since a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, even when the first light source and the second light source are unitized, and positions of the two light sources are fixed to prevent the two light sources from moving separately in the direction that is in parallel with an optical axis of the light-converging optical system, it is possible to correct chromatic aberration with a diffraction effect of the diffraction pattern, and thereby to conduct excellent recording/reproducing of information. Further, since the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis, even when the light source on one side is arranged at the position where the light flux is not emitted in the direction that is in parallel with an optical axis of the light-converging optical system, the light flux can be emitted in the direction of the optical axis as if such light source is on the optical axis of the light-converging optical system, resulting in better recording/reproducing of information.
Structure (69) is represented by a coupling lens which is a single lens.
Structure (70) is represented by an optical pickup apparatus coupling lens which has, on its one surface only, a diffraction pattern, and an effect of diffraction of the diffraction pattern makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system to emerge in the direction that is almost in parallel with the optical axis.
Structure (71) is represented by an optical pickup apparatus coupling lens wherein diffraction patterns are provided on both sides of the coupling lens, and an effect of diffraction on the both sides corrects chromatic aberration, and makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system to emerge in the direction that is almost in parallel with the optical axis.
Structure (72) is represented by an optical pickup apparatus coupling lens wherein diffraction patterns are provided on both sides of the coupling lens, and an effect of diffraction of the diffraction pattern on one side corrects chromatic aberration, and an effect of diffraction of the diffraction pattern on the other side makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system to emerge in the direction that is almost in parallel with the optical axis.
Structure (73) is represented by an optical pickup apparatus coupling lens for recording/reproducing of information which has therein a light-converging optical system having therein a coupling lens which changes the state of divergence for light fluxes emitted from the first light source with wavelength xcex1 and the second light source with wavelength xcex2 and an objective lens for converging the light flux whose state of divergence has been changed on the recording surface of an optical information recording medium, and a detector for detecting reflected and/or transmitted light from the recording surface, and can record and/or reproduce information for the first and second optical information recording media each having a different thickness of a transparent base board, wherein the first light source and the second light source are unitized, and a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, and the coupling lens has off-axis characteristics to correct off-axis coma in the light-converging optical system for the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source.
Since a diffraction pattern that corrects chromatic aberration of each of a light flux emitted from the light source with wavelength xcex1 and a light flux emitted from the light source with wavelength xcex2 is formed on at least one surface of the coupling lens, even when the first light source and the second light source are unitized, and positions of the two light sources are fixed to prevent the two light sources from moving separately in the direction that is in parallel with an optical axis of the light-converging optical system, it is possible to correct chromatic aberration with a diffraction effect of the diffraction pattern, and thereby to conduct excellent recording/reproducing of information. Further, since the coupling lens makes a principal ray of light of the light flux incident obliquely from an optical axis of the light-converging optical system among light fluxes emitted from the first light source or the second light source to emerge in the direction that is almost in parallel with the optical axis, even when the light source on one side is arranged at the position where the light flux is not emitted in the direction that is in parallel with an optical axis of the light-converging optical system, the light flux can be emitted in the direction of the optical axis as if such light source is on the optical axis of the light-converging optical system, resulting in better recording/reproducing of information. Furthermore, coma on a certain level is caused by passage through an objective lens and a transparent base board on off-axis light which causes a certain image height. Therefore, if it is possible to give inverse coma so that the coma may be canceled by the coupling lens, it is possible to reduce coma of the total light-converging optical system effectively for the light flux incident obliquely from the optical axis of the light-converging optical system, and thereby, to conduct excellent recording/reproducing of information.
Structure (74) is represented by an optical pickup apparatus coupling lens wherein the wavelength xcex1 and the wavelength xcex2 have the relationship of xcex1 less than xcex2, and the relationship of t1 less than t2 holds when a thickness of a transparent base board of the first optical information recording medium is represented by t1 and a thickness of a transparent base board of the second optical information recording medium is represented by t2, thus, recording/reproducing of information can be conducted on different recording media such as, for example, CD and DVD.
Structure (75) is represented by an optical pickup apparatus coupling lens wherein a light flux emitted from the second light source enters obliquely for an optical axis of the light-converging optical system.
Structure (76) is represented by an optical pickup apparatus coupling lens wherein the coupling lens is a collimator which converges a incident divergent light flux to be a collimated light flux which is almost in parallel with an optical axis.
Structure (77) is represented by a coupling lens that changes the state of divergence of a light flux, wherein a principal ray of light of the light flux incident obliquely from an optical axis of the coupling lens is made to emerge in the direction that is almost in parallel with the optical axis of the coupling lens.
Since the coupling lens is arranged to make a principal ray of light of the light flux incident obliquely from an optical axis of the coupling lens to emerge in the direction that is almost in parallel with the optical axis, even when the light source is arranged at the position where the light flux is not emitted in the direction that is in parallel with the optical axis of the coupling lens, light-converging optical system, the light flux can be emitted in the direction of the optical axis as if such light source is on the optical axis of the light-converging optical system, resulting in better recording/reproducing of information in an optical pickup apparatus.
Structure (78) is represented by a coupling lens wherein a diffraction pattern is formed on at least one surface of the coupling lens.
Structure (79) is represented by a coupling lens changing the state of divergence of a light flux wherein a diffraction pattern that corrects chromatic aberration in light fluxes emitted from at least two light sources each having a different wavelength is formed on at least one surface of the coupling lens, and among two light fluxes each having a different wavelength, a principal ray of light of the light flux incident obliquely from an optical axis of the coupling lens is made to emerge in the direction that is almost in parallel with the optical axis of the coupling lens.
Since a diffraction pattern that corrects chromatic aberration of light fluxes emitted from light sources each having a different is formed on at least one surface of the coupling lens, even when positions of the two light sources are fixed to prevent the two light sources from moving separately in the direction that is in parallel with an optical axis of the coupling lens, it is possible to correct chromatic aberration with a diffraction effect of the diffraction pattern, and thereby to conduct excellent recording/reproducing of information. Further, since the coupling lens is arrange to make a principal ray of light of the light flux incident obliquely from an optical axis of the coupling lens among light fluxes emitted from light sources each having a different wavelength to emerge in the direction that is almost in parallel with the optical axis, even when the light source on one side is arranged at the position where the light flux is not emitted in the direction that is in parallel with an optical axis of the light-converging optical system, it is possible to emit the light flux in the direction of the optical axis as if such light source is on the optical axis of the light-converging optical system, and thereby to conduct better recording/reproducing of information.
Structure (80) is represented by a coupling lens which is a single lens.
Structure (81) is represented by a coupling lens wherein only one surface of the coupling lens is provided with a diffraction pattern, and a diffraction effect of the diffraction pattern makes a principal ray of light of the light flux incident obliquely from an optical axis of the coupling lens to emerge in the direction that is almost in parallel with the optical axis of the coupling lens.
Structure (82) is represented by a coupling lens wherein both surfaces of the coupling lens are provided with a diffraction pattern which corrects chromatic aberration, and a diffraction effect of the diffraction patterns on the both surfaces makes a principal ray of light of the light flux incident obliquely from an optical axis of the coupling lens to emerge in the direction that is almost in parallel with the optical axis of the coupling lens.
Structure (83) is represented by a coupling lens wherein both surfaces of the coupling lens are provided with a diffraction pattern, and chromatic aberration is corrected by a diffraction effect of the diffraction pattern on the surface on one side, while a diffraction effect of the diffraction pattern on the surface on the other side which corrects chromatic aberration, and a diffraction effect of the diffraction patterns on the both surfaces makes a principal ray of light of the light flux incident obliquely from an optical axis of the coupling lens to emerge in the direction that is almost in parallel with the optical axis of the coupling lens.